1. Field of the Invention
The present invention relates to hybrid automotive power-trains, specifically in the area of vibration control.
2. Background Art
Torsional resonance vibration has always been an inherent problem with automobiles powered by an internal combustion engine because of engine inertia forces. Methods that have been devised to minimize this vibration usually involve adding a damper system, which uses an additional mass to absorb vibration forces.
The added mass of such damper systems minimizes vibrations, but it also adds extra weight to the automobile. The extra weight of a traditional damper system affects the automobile's performance and adds to its complexity.
The torsional resonance vibration is diminished in hybrid vehicles that are powered by an internal combustion engine and an electrical motor, but it is not eliminated. By tradition, the torsional vibration problem in a hybrid vehicle has been dealt with in the same way as in an internal combustion engine vehicle; i.e., by adding an extra mass to absorb vibration forces, even though a hybrid vehicle is built quite differently than a gasoline-powered automobile.
Generally, a hybrid electric vehicle combines an electric power source with a traditional internal combustion engine power source to achieve enhanced fuel economy and lower exhaust emissions. Electric propulsion typically has been generated through the use of batteries and electric motors. Such an electric propulsion system provides desirable characteristics of high torque at low speeds, high efficiency, and an opportunity to regeneratively capture otherwise lost braking energy.
Propulsion using an internal combustion engine provides high energy capability and enjoys an existing infrastructure and lower cost due to economics of scale. By combining the two power sources with a proper control strategy, the result is a reduction in the use of each power source in its less efficient range. Furthermore, in the case of a parallel hybrid configuration, the combination of a down-sized engine with an electric propulsion system results in better utilization of the engine, which improves fuel consumption. The electric motor and battery can compensate for reduction in the engine size.
In typical configurations, the combination of the two types of propulsion systems (internal combustion engine and electric) is usually characterized as either series or parallel hybrid system. In a pure series hybrid propulsion system, only the electric motor(s) is in direct connection with the drivetrain and the engine is used to generate electric power that is fed to the electric motor(s). The advantage of this type of system is that the engine can be controlled independently of driving conditions and therefore can be consistently run with optimum fuel efficiency and low emission levels. A key disadvantage to the series arrangement is the loss of energy experienced because of the inefficiencies associated with full conversion of mechanical engine output energy to electrical energy.
In a pure parallel hybrid propulsion system, both the engine and the electric motor are directly connected to the drivetrain, and either one may independently drive the vehicle. Because there is a direct mechanical connection between the engine and the drivetrain in a parallel hybrid propulsion system, less energy is lost through conversion to electricity compared to a series hybrid propulsion system. The operating point for the engine, however, cannot always be chosen with full freedom.
The two hybrid propulsion systems can be combined into either a switching hybrid propulsion system or a parallel-series hybrid propulsion system. A switching hybrid propulsion system typically includes an engine, a generator, a motor, and a clutch. The engine is typically connected to the generator. The generator is connected through a clutch to the drivetrain. The motor is connected to the drivetrain between the clutch and the drivetrain. The clutch can be operated to allow series or parallel hybrid propulsion.
A parallel-series hybrid system, as in the case of the present invention, includes an engine, a generator, and a motor. A planetary gear set allows a series power flow path from the engine to the generator and a parallel power flow path from the engine directly to the drivetrain. In a parallel-series hybrid system, the engine speed can be controlled by way of the series power flow path, while maintaining a mechanical connection between the engine and drivetrain through the parallel path. The motor augments the engine power in the parallel path, as in the case of a traction motor in a pure parallel hybrid propulsion system, and provides an opportunity to use energy directly through the series path, thereby reducing the losses associated with converting the electrical energy into and out of chemical energy from the battery.
In a typical parallel-series hybrid system, the generator is connected to the sun gear of the planetary gear set. The engine is connected to the planetary carrier. The output gears, usually including an output shaft and gears for connecting the motor and the final drive, are connected to the ring gear. In such a configuration, the parallel-series hybrid system generally operates in four different modes; one electric mode and three hybrid modes.
In the electric mode, the parallel-series hybrid system propels the vehicle using only stored electrical energy and the engine is turned off. The tractive torque is supplied from the motor, the generator, or a combination of both. This is the preferred mode when the desired power is low enough so that it can be produced more efficiently by the electrical system than by the engine when the battery is sufficiently charged. This is also a mode for reverse driving because the engine cannot provide reverse torque to the powertrain in this configuration.
In the parallel hybrid mode, the engine is operating and the generator is locked. By doing this, a fixed relationship between the speed of the engine and the vehicle speed is established. The motor operates as a motor to provide tractive torque to supplement the engine's power, or it can be operated to produce electricity as a generator. This mode is used whenever the required power demand requires engine operation and the required driving power is approximately equal to an optimized operating condition of the engine. This mode is especially suitable for cruising speeds. It is maintained by a small internal combustion engine fitted to the hybrid electric vehicle.
In a parallel-series hybrid mode, the engine is on and its power is divided between a direct mechanical path to the drivetrain and an electrical path through the generator. The engine speed in this mode is typically higher than the engine speed in the parallel mode, thus effecting higher engine power. The electrical energy produced by the generator can flow to the battery for storage or to the motor for immediate use. In the positive parallel-series mode, the motor can be operated as either a motor to provide tractive torque to supplement the engine's power or to produce electricity in combination with the generator. This is the preferred mode whenever high engine power is required for tractive powering of the vehicle, such as when high acceleration is called for; e.g., in passing or uphill ascents. This is a preferred mode used when the battery is charging.
In a negative parallel-series hybrid mode, the engine is in operation and the generator is used as a motor acting against the engine to reduce its speed. Consequently, engine speed, and therefore engine power, is lower than engine speed in a parallel mode. If needed, the motor can also be operated to provide tractive torque to the drive-train or to generate electric power therefrom. This mode is typically never preferred due to increased losses at the generator and planetary gear system, but it will be utilized when engine power is required to be decreased below that which would otherwise be produced in parallel mode. This situation will typically be brought about because the battery is in a well-charged condition and there is low tractive power demand. In this regard, whether operating as a generator or motor, the torque output of the generator is always of the same sense (+/−); that is, the torque is always directionally opposed to that of the engine. The sign of the speed of the generator, however, alternates between negative and positive values depending upon the direction of rotation of its rotary shaft, which corresponds to the generator mode versus the motor mode. Because power is dependent upon the sense of the speed (torque remains of the same sense), the power will be considered to be positive when the generator is acting as a generator and negative when the generator is acting as a motor.
When slower engine speed is desired, the current supplied to the generator is changed, causing the speed of the generator to slow. This in turn slows the engine. This effect is accomplished because the resistive force acting against the torque of the generator is less at the engine than at the driveshaft, which is connected to the wheels and is influenced by the entire mass of the vehicle. It should be appreciated that the change in speed of the generator is not equal, but instead proportional to, that of the engine because of gear ratios involved in the connection therebetween.
Typically, to achieve a smooth engine start in a hybrid electric vehicle in which the engine is mechanically interconnected to the drive wheels, the start of engine fuel injection and ignition is made at speeds above any mechanical resonance speeds of the drivetrain. Additionally, at full take-off acceleration, any delay in the engine's production of power typically decreases engine performance. Still further, to achieve smooth driving characteristics and obtain low fuel consumption, the engine torque and speed change rates must be limited. At full take-off, this usually results in an increased time for the engine to reach maximum power, and all of these conditions deteriorate acceleration performance of the vehicle.
As can be appreciated, the engine is not always running during vehicle operation. If the engine is stopped for a sufficiently long period during operation of the vehicle, the exhaust system catalyst may cool down to such a degree that a temporary, but significant, increase in exhaust emissions may occur upon restart until the catalyst once again warms to its effective temperature.
In a typical parallel-series hybrid electric propulsion arrangement, the control strategy involves operating the engine along optimum efficiency torque versus speed curves. A trade-off exists between traction force performance and fuel economy that, for optimization, typically requires selection of a particular gear ratio between the engine and the wheels that causes the engine to deliver more power than is needed for vehicle propulsion. This generally occurs during cruising in parallel mode, or near constant vehicle velocity conditions. Operation under these conditions can sometimes cause the battery and charging system to reject energy being delivered thereto from the engine. This problem is generally solved by decreasing or limiting the engine output power by entering negative split mode that entails using the generator as a motor to control the engine to a decreased speed. Such control allows the engine to follow an optimum curve at reduced engine output power.
Use of the generator as a motor gives rise to power circulation in the powertrain, which leads to undesirable energy losses at the generator, motor, inverters and planetary gear set. These energy losses may be manifest as heat generation, which indicates that most efficient use is not being made of the installed drivetrain.
In a parallel-series hybrid propulsion system having one or more planetary gear sets and utilizing a generator lock-up device, harshness in ride occurs when the generator lock-up device is engaged and released. This is due primarily to the difference in how the engine torque is estimated when the vehicle operates in different operating modes. Typically, when the generator is locked-up, engine torque is estimated from the combustion control process of the engine. When the generator is free, however, as in a parallel-series mode, engine torque is estimated from the generator torque control process. The difference in values of these two estimating techniques gives rise to what usually amounts to a variation in operating torque between the engine and generator when the lock-up device is engaged or disengaged, thereby creating harshness in the vehicle's operation, usually manifest as abrupt changes or jerkiness in the vehicle's ride.
The generator is typically used to control the engine in a parallel-series hybrid mode. This is usually accomplished by employing a generator having maximum torque capabilities substantially greater than the engine's maximum torque that is transmittable to the planetary gear system. Failure to have such a control margin can result in generator over-speed and possible damage to the propulsion system. Such a control margin means, however, that the engine and generator are not fully exploited at full capacity acceleration.
There are several deficiencies associated with the use of known hybrid electric vehicle designs described above, and one of them is related to torsional resonance vibrations.
Torsional vibration is caused, among other reasons, by the unevenness of crankshaft rotation for an internal combustion engine and consequent rotation of the drivetrain. The torsional vibration may comprise an entire spectrum of vibrations of different frequencies and may resonate with the natural frequency of the body of a vehicle. The torsional resonance vibrations that are in the driving range create a vibration or a noise that is objectionable to drivers and passengers.
One way to move these resonance vibrations out of a critical driving range is the use of an auxiliary damper, usually located on the driveshaft, which is commonly known as prop-shaft damper. This auxiliary damper comprises a torsional spring and a mass. It can be tuned to a specific frequency. This combination of torsional springs and masses adds weight to a vehicle and increases its cost. The added weight has a direct adverse effect on the fuel consumption.
The additional weight is especially undesirable in hybrid vehicles because of limited power provided by the electrical motor. Hybrid vehicles tend to be less heavy when compared to a vehicle propelled by a traditional internal combustion engine, so the hybrid vehicles can have better performance with an electrical motor. Any additional weight will affect this performance objective.
Therefore, a better solution to this torsional resonance vibration is clearly needed.